Mobile offshore drilling unit

ABSTRACT

A mobile offshore drilling unit is described that has an operating deck supported on a submersible deck, which may be a pontoon deck or a mat deck, by four or more columns, wherein a ratio of displacement of the mobile offshore drilling unit to total column cross-sectional area is at least about 40 mT/m 2  during operation. The submersible deck has a recess at each end with a thruster group disposed in each recess. The thrusters of the thruster group are displaced in a longitudinal direction and a transverse direction from each other.

FIELD

Embodiments described herein relate to a vessel for supporting offshore drilling activities. More specifically, embodiments of semi-submersible vessels for drilling and well intervention are disclosed.

BACKGROUND

Production of oil and gas is a trillion dollar industry. In 2010, over 28 percent of the oil and 15 percent of the natural gas produced in the United States was produced in the Gulf of Mexico at a cost of some $25 billion. Globally, between 2010 and 2015, the offshore oil and gas industry is expected to spend about $200 billion per year in investment and operating expense. This large, dynamic industry requires utility vessels capable of operating in a variety of water depths and conditions to provide drilling and/or well-intervention capabilities. Such capabilities include altering the state of the well hardware or its geometry, performing well diagnostic operations, or managing production of the well.

Typically referred to as mobile offshore drilling units, or MODU's, important capabilities for such vessels include stability while floating in a variety of ocean conditions, efficient transportability in or out of water, and compatibility with key strategic waterways such as the Panama Canal and the Suez Canal, along with versatile deck fitting, crew accommodations, and utility spaces. There is a continuing need for MODU's having improved features.

SUMMARY

Embodiments disclosed herein provide a mobile offshore drilling unit with an operating deck supported on a submersible deck by four or more columns, wherein a ratio of displacement of the mobile offshore drilling unit to total column cross-sectional area is at least about 40 mT/m² during operation. The submersible deck has a recess at each end with a thruster group disposed in each recess. The thrusters of the thruster group are displaced in a longitudinal direction and a transverse direction from each other.

The submersible deck may be a pontoon deck or a mat deck. The operating deck may be attached to a platform integral to the columns. The operating deck is typically a box deck that may support equipment typical of a well intervention vessel or a drill ship without redesign of the hull or deck. A gantry is positioned over an opening in the operating deck that provides access to lower decks and to the water surface. One or two cranes may be supported on the operating deck, along with a pipe rack and laydown area. The operating deck may feature a helideck and living quarters and/or control center.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is a side view of a mobile offshore drilling unit (“MODU”) according to one embodiment.

FIG. 2A is an end view of a MODU according to another embodiment.

FIG. 2B is a bottom view of a portion of the MODU of FIG. 2A.

FIG. 2C is a cross-sectional view of a portion of the MODU of FIG. 2A.

FIG. 3 is a top view of the MODU of FIG. 2A.

FIG. 4 is a side view of a MODU according to another embodiment.

FIG. 5 is an end view of a MODU according to another embodiment.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DETAILED DESCRIPTION

FIG. 1 is a side view of a mobile offshore drilling unit (“MODU”) 100 according to one embodiment. The MODU 100 is semi-submersible, with a submersible deck 102 shown below the datum 1 representing a water surface. The MODU 100 has an operating deck 104 supported from the submersible deck 102 by a plurality of columns 106.

Typically at least four columns 106 support the operating deck 104 above the submersible deck 102, but column count may range from four to eight. The column height 160 is selected to provide a desired maximum draft, typically the operating draft, while also providing deck clearance above the maximum wave height. In the embodiment of FIG. 1, the column height 160 is between about 15 m and about 20 m, for example about 18.5 m. The columns are spaced apart by a distance selected to provide stability of the vessel. In the embodiment of FIG. 1, the columns are aligned in the longitudinal and transverse directions. That is, a line or plane that intersects the center line of each of the columns visible in FIG. 1 is parallel to a center line longitudinal axis of the vessel. Likewise, the columns are aligned in a direction perpendicular to the center line longitudinal axis of the vessel. The columns may be aligned differently, however. For example, the columns may be staggered in the transverse direction and/or the longitudinal direction, so long as vessel stability is maintained.

Each of the columns 106 has a transverse dimension and a longitudinal dimension, and the transverse dimension is less than the longitudinal dimension. The column dimensions are selected to provide a total column cross-sectional area that enhances vessel stability. A ratio of vessel displacement to total column cross-sectional area at operating conditions is typically at least about 40 mT/m², such as between about 40 mT/m² and about 60 mT/m², for example about 45 mT/m² for a heave characteristic period of about 19 seconds to about 35 seconds, for example about 20 seconds. The columns 106 of FIG. 1 have a transverse dimension of about 13 m and a longitudinal dimension of about 14 m. Each column has a tapered profile to reduce hydrodynamic drag while the vessel is moving, thus improving energy efficiency. The tapered portions may be angled or rounded, or a combination thereof. In the embodiment of FIG. 1, the tapered portions form an angle of about 45% with respect to the longitudinal and transverse directions.

The submersible deck 102 may be a mat deck or a plurality of pontoons, usually two pontoons. A pontoon deck provides better station-keeping and faster transit due to a small effective hydraulic area, and the lower overall weight of a pontoon design allows for use of a DP2 thrust system. A mat hull, however, provides advantages in roll stability and vessel motion characteristics, and allows for construction of a narrower vessel overall. The pontoons and mat hull are typically hollow to reduce weight and to provide a space for placing operating equipment such as ballast pumps and/or thruster control equipment. The pontoons and mat hull may be double-hulled or double-skinned. The pontoons feature rounded surfaces to reduce hydrodynamic drag and increase structural strength, since rounded surfaces do not introduce stress points in the manner of corners.

The submersible deck 102 has a central portion 166 and two end portions 162. The submersible deck 102 has a thickness at the central portion 166 that is greater than a thickness at the two end portions 162, so that each of the end portions 162 defines a recess 108, wherein a lower surface 164 of the submersible deck 102 in each recess 108 is above a bottom surface 120 of the submersible deck 102 at the central portion 166 thereof. The recesses 108 have a height 119 above the bottom surface 120 of the submersible deck 102 that is selected to provide space to mount one or more thruster groups 112. The lower surface 164 of each recess 108, on which the thruster groups 112 are mounted, typically has an elevation above the bottom surface 120 of the submersible deck between about 3 m and about 10 m, for example about 5 m.

Each thruster group 112 has a plurality of thrusters 114 mounted to the lower surface 164 of the recess 108. The thrusters 114 protrude below the bottom surface 120 of the submersible deck 102 by a distance 118 that is reduced by the height 119 of the recess 108. The vessel 100 is therefore able to operate in shallow water, and transporting the vessel 100 requires less platforming because the thrusters extension is reduced. The recess height 119 is selected to provide a desired extension of the thrusters below the bottom surface 120 of the submersible deck 102 such that water flow into the thrusters, and energy efficiency of the thrusters, is not substantially impaired. The recess height 119 may be between about 1 m and about 6 m, such as between about 1.5 m and about 4 m, for example about 2 m. The recesses also reduce drag under towing. The recess length along the longitudinal direction of the MODU, typically less than about 35 m, such as between about 5 m and about 30 m, for example about 20 m, is selected to provide space for rotation of the thrusters 114 and for sufficient flow field for the thrusters 114 to operate efficiently. The recesses may be the same length at each end, or different lengths.

Each of the columns 106 has an extremity 168 that is aligned with the point at which the end portion 162 of the submersible deck 102 meets the central portion 166. Such an arrangement simplifies construction of the vessel 100, but is not critical to the operation thereof. The columns 106 may be displaced from the junction of the end portion 162 and the central portion 166, if desired, so long as mechanical stresses on the end portions 162 and the junction thereof with the central portion 166 are adequately managed.

The submersible deck 102 extends beyond the operating deck 104 by no more than about 10 m at either end. The columns 106 may be coterminous with the operating deck 104, or the, operating deck may extend beyond the columns 106 in the longitudinal and/or transverse directions. In FIG. 1, the operating deck 104 is shown extending beyond the columns 106 in the longitudinal direction by about 2 m. Placement of the columns 106 with respect to the operating deck 104 depends on arrangement of load on the operating deck 104 and stability considerations. In the embodiment of FIG. 1, extension of the operating deck 104 beyond the columns 106 allows for slightly larger operating area, and the extension is enabled by the stability of the column cross-sectional area.

The operating deck 104 is a box deck structure comprising a plurality of decks. The operating deck 104 may be attached to the columns 106 directly, or as shown in FIG. 1, the operating deck 104 may be attached to a platform 116 formed as an integral part of the column structure or attached to the columns 106. The platform 116 may be useful in some embodiments for promoting structural rigidity while enabling movement of the columns 106 to the extremities of the operating deck 104. If event the submersible deck 102 is a mat deck, the mat deck provides a spacious work area when the vessel is at transit elevation, indicated by datam level 2.

The overall length of the MODU 100 of FIG. 1 is about 104 m, and the operating deck 104 is 34 m above the bottom surface 120 of the submersible deck 102. The operating deck is about 84 m in length. The vessel has a positive metacentric height during all modes of operation, with a minimum GM of 2.0 m for stable sailing in moderate conditions, but alternate embodiments may be made having GM of zero or more. The MODU 100 may be scaled up to 20% smaller or larger with substantially the same stability. The MODU 100, equipped with a dual pontoon submersible deck, and operating at a draft of about 54 ft. with eight 3 MW DP2 thrusters, four at each end, is expected to maintain station at all orientations in winds up to about 40 knots and surface currents up to about 3 knots. The MODU 100 may be completed at a displacement of 20,000 tonnes with a transit draft less than about 10 m, such as less than about 8 m, for example about 7 m.

Equipment on the operating deck 104 may be configured according to specification. In the embodiment of FIG. 1, a centrally positioned gantry 152 is flanked by a 75 ton crane 154. A helideck 156 is located at one end of the operating deck 104 near a quarters 158 and bridge structure 156. The operating deck 104 of the MODU 100 in FIG. 1 has about 3000 m² of clear space.

FIG. 2A is an end view of a MODU 200 according to another embodiment. The MODU 200 features a pontoon embodiment with two pontoons 202. Each pontoon 202 has a thruster group 112, each thruster group having two thrusters 114A and 114B. The thrusters 114A/B of a single thruster group 112 are positioned with a displacement in the transverse and the longitudinal direction. FIG. 2B is a bottom view of one pontoon 202 showing the transverse and longitudinal displacement of the two thrusters 114A/B with respect to the transverse direction 240 and the longitudinal direction 230. The displacement shown in FIG. 2B streamlines water flow through the thrusters 114A/B and reduces energy consumption.

The columns 106 are separated by a gap 206 set by the column dimensions and placement to provide a desired stability of the MODU. In the end view of FIG. 2A, the operating deck 104 has a width of about 61 m, and the gap between the columns 106 is about 35 m. The pontoons 202 are wider than the columns 106 to provide ample clearance between the thrusters 114 in each thruster group 112, and to enhance stability. The pontoons 202 in FIG. 2A have a width of about 16 m each, and a center line or plane of each pontoon 202 parallel to the longitudinal direction 230 is aligned with a center line or axis of the corresponding column 106. In other embodiments, however, the pontoons 202 may be displaced with respect to the columns 106, either toward the central axis of the MODU 200 or away from the central axis of the MODU 200 to adjust stability and subsurface width as needed.

FIG. 2C is a cross-sectional view of one of the columns 106. Each column 106 has an interior space 222 allowing for ballast to control elevation of the operating deck 104 above the water surface. In operating mode, the MODU 100 or 200 may have a displacement that is at least one-third ballast, while in transit mode the displacement will typically be less than about 10% ballast. The interior space 222 may also allow for passageways to provide access from the operating deck 104 to the pontoons 202, or to a mat deck in such an embodiment. The walls 220 of each column 106 may be double-hulled or double-skinned to enhance water exclusion from passageways and equipment areas. Each of the columns 106 has tapered corners 224 to enhance hydrodynamics, increasing transit speed, improving station keeping, and reducing resistance to towing. In the embodiment of FIG. 2A, the corners 224 are tapered to a 45° angle, but the corner may be tapered to any angle to provide a desired hydrodynamic property. Typically, the corners 224 are tapered to an angle between about 30° and about 60°, and the angle may be the same or different from corner to corner. The corners may also be rounded, rather than tapered, or any mixture or combination of rounding and tapering. The corners may be contoured in the same way or in different ways from corner to corner.

The tapered corners may have any desired length defining the degree to which the corners are tapered. For example, if the exemplary column of FIG. 2C has a dimension 228 of 14 m, a distance 230 from a center line 226 of the column to a corner taper may be between about 4 m and about 6 m, for example about 5.5 m.

The MODU 200 of FIG. 2A features a second crane 210 for added utility. The various embodiments of MODU vessels described herein, such as the MODU 100 and the MODU 200, may be converted to drill ships with no redesign of the deck or hull, because the hull and deck designs described herein have sufficient stability under the added load of equipment normally specified for a drill ship.

FIG. 3 is a top view of a MODU 300 according to another embodiment. The MODU 300 of FIG. 3 features the centrally located gantry 152 adjacent to a pipe rack 302 and a large laydown area 304. An opening 306 in the operating deck 104 leads to subdeck operating structures including a drill floor and moon pool under the gantry 152. The MODU 300 incorporates the hull structure and stability features described in connection with FIGS. 1 and 2A, enabling movement of cranes and other large footprint equipment to the periphery of the operating deck 104, leaving substantial space for work activity.

FIG. 4 is a side view of a MODU 400 according to another embodiment. The MODU 400 is similar to the MODU vessels 100, 200, and 300, with the addition of an additional column 410 or pair of columns between the columns 106. The column 410, or column pair comprising the column 410, may be included to add support strength in the event deck weight is increased. The columns may also be included to improve rigidity and/or stability in the event the MODU is lengthened. Whereas the MODU's 100, 200, and 300 generally have a ratio of length to width between about 1.5 and 1.8, for example about 1.7, the MODU 400 may have a ratio of length to width between about 1.5 and about 2.0, for example about 1.9.

FIG. 5 is an end view of a MODU 500 according to another embodiment. The MODU 500 features a mat deck 502 with three thrusters 504, each of which may also be a thruster group comprising two or more thrusters. A mat deck enables providing a thruster 504 in a central location additional to the thrusters 504 at the peripheral locations, if desired, to improve maneuverability. The mat deck 502 features an opening (not shown) that allows water access from the gantry. The mat deck 502 offers increased structural strength that allows a narrower vessel. The embodiment of FIG. 5 may have a ratio of length to width between about 1.5 and about 2.5, for example about 2.4. In one embodiment, the MODU 500 has a width no more than about 48 m, enabling passage through the Panama Canal.

While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. 

What is claimed is:
 1. A mobile offshore drilling unit, comprising: an operating deck supported on a submersible deck by four or more columns having a total cross-sectional area, wherein a ratio of displacement of the mobile offshore drilling unit to total column cross-sectional area is at least about 40 mT/m² during operation.
 2. The mobile offshore drilling unit of claim 1, wherein the submersible deck has a first end and a second end, each of the first end and the second end having at least two thruster groups, each thruster group comprising at least two thrusters.
 3. The mobile offshore drilling unit of claim 2, where each thruster of each thruster group is displaced along a longitudinal axis of the mobile offshore drilling unit and along a transverse axis of the mobile offshore drilling unit with respect to the other thrusters of the thruster group.
 4. The mobile offshore drilling unit of claim 2, wherein no thruster protrudes below the submersible deck by more than about two meters.
 5. The mobile offshore drilling unit of claim 2, wherein each thruster delivers power of at least about 3 MW.
 6. The mobile offshore drilling unit of claim 1, wherein the submersible deck extends beyond the operating deck no more than about ten meters.
 7. The mobile offshore drilling unit of claim
 3. wherein each column has a dimension along the longitudinal axis greater than a dimension along the transverse axis.
 8. The mobile offshore drilling unit of claim 4, wherein the operating deck is a box deck.
 9. The mobile offshore drilling unit of claim 8, wherein the mobile offshore drilling unit has an operating displacement that is at least one-third ballast.
 10. The mobile offshore drilling unit of claim 1, wherein the mobile offshore drilling unit has a horizontal dimension no more than about 48 meters.
 11. The mobile offshore drilling unit of claim 3, wherein the submersible deck is a pontoon deck.
 12. A mobile offshore drilling unit comprising: an operating deck having a box structure; a submersible deck below the operating deck; a plurality of columns supporting the operating deck above the submersible deck; a recess at opposite ends of the submersible deck; and a thruster group in each recess of the submersible deck.
 13. The mobile offshore drilling unit of claim 12, wherein each thruster group comprises a plurality of thrusters, and each thruster in a thruster group is displaced with respect to every other thruster of the thruster group along at least two orthogonal axes of the mobile offshore drilling unit.
 14. The mobile offshore drilling unit of claim 13, wherein the columns have a total cross-sectional area, and a ratio of displacement of the mobile offshore drilling unit to the total cross-sectional area of the columns is at least about 40 mT/m² during a well operation.
 15. The mobile offshore drilling unit of claim 14, wherein the submersible deck is a pontoon deck or a mat deck.
 16. The mobile offshore drilling unit of claim 15, wherein the mobile offshore drilling unit has a transverse dimension not more than about 48 meters.
 17. The mobile offshore drilling unit of claim 15, wherein the thrusters protrude below a bottom surface of the submersible deck by no more than about two meters.
 18. The mobile offshore drilling unit of claim 17, wherein the columns have a tapered cross-section.
 19. The mobile offshore drilling unit of claim 15, wherein the columns support a platform integral to the columns, and the operating deck is attached to the platform.
 20. A mobile offshore drilling unit, comprising: a submersible deck having a recess in a lower surface at each end; two or more thruster groups in each recess of the submersible deck, each thruster group comprising two or more thrusters, wherein each thruster of a thruster group is displaced from every other thruster in the thruster group in a longitudinal direction and in a transverse direction of the mobile offshore drilling unit, and the thrusters protrude below the lower surface of the submersible deck by no more than about two meters; an operating box deck supported above the submersible deck by a plurality of hollow rectangular columns with tapered corners, the hollow columns having a total cross-sectional area such that a ratio of displacement of the mobile offshore drilling unit during operation to the total cross-sectional area of the columns is at least about 40 mT/m²; and a platform integral to the columns, to which the operating box deck is attached.
 21. The mobile offshore drilling unit of claim 20, wherein a lower surface of each recess has an elevation above a bottom surface of the submersible deck between about 3 m and about 10 m, and the columns are integral to the submersible deck.
 22. The mobile offshore drilling unit of claim 21, wherein a transit draft of the MODU is less than about 10 m, and a length of each recess is between about 5 m and about 30 m. 